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The Southern Hemisphere's only population of the mysterious sockeye salmon has started its spawning run in the MacKenzie Country's alpine rivers, providing the region with a new tourist attraction.

In the Twizel River, the sockeye are passing under the State Highway 8 Bridge near Twizel township, and the sight of hundreds of fish moving up the river, and spawning right below the bridge, has become popular with tourists and locals.

The chance to see these fish migrate leads to dozens of people crowding the bridge to look at them.

The sockeye are highly visible, often bursting out of the water in a shower of spray as they scramble across the riffles in their search for the ideal spot in the riverbed to lay their eggs.

Released in 1901 as an attempt to create a sea-run salmon canning industry, the attempt failed when the sockeye never ran to the sea, leaving the Chinook salmon to become the basis for the South Island's successful salmon fishery.

Sockeye were thought to have died out in the late 1980s. However around 2005, Central South Island Fish & Game started receiving reports of them spawning once again.

Now the sockeye can be found in their thousands heading upstream at this time of year to breed.

"This year the spawning effort appears to be wide spread and the numbers are reasonably high," he said.

"Sockeye have been observed in almost all of the Lake Benmore tributary rivers and streams, most notably the upper Ahuriri River and its tributaries, Lower Ohau River and its tributaries like the Twizel and Fraser Rivers, the Tekapo River and its tributaries like the Mary Burn and Forks River".

Mr Couper said sockeye were also turning up in areas where they were not thought to exist.

"The other interesting observation this year is the Lake Pukaki population is flourishing, yet Fish & Game staff only heard about them existing there last year," he said.

"There had not been any confirmed reports of sockeye in Lake Pukaki for decades, now this year there are around 1000 spawning fish in just one of the lake's tributary streams."

Mr Couper warned people against disturbing the fish.

"It is worth noting that it is an offence under the Conservation Act to disturb spawning salmon - you can't catch, net or spear the fish, or even walk in the river bed and trample their redds, or nests."

This was why the Twizel bridge site was so popular, as people could watch the fish from above without disturbing them.